The present invention relates to a method of measuring the exact position of the energy centre of an image spot of a bright object on a photosensitive detector constituted by a charge transfer detection matrix, said image spot being defocused to allow a subsequent barycentric calculation mode of said exact position and the luminous signal being digitized and thereafter processed in the form of digital pixels.
A charge transfer detection matrix is a device having a photosentive mosaic which, in response to an incident light ray, produces a video signal consisting of a pulse train whose individual amplitudes are proportional to the quantity of energy of the photons in each sensitive element of the device. Such a device is used more specifically in mosaic detectors employed in astronomical view finders and in space technology as a camera for the photography of (high magnitude) weak stars.
The mosaic detectors comprising a large number of new-generation sensitive elements are combined with high-speed microprocessors to provide star-tracking arrangements and instruments capable of tracking an apparently fast moving star. In these arrangements, which are intended for space missions, it is essential that the position of the energy centre of an image spot on the detection matrix be determined and recorded with high precision, in fact a precision equal to a fraction of the picture element or pixel. Such an accurate detection typically having an accuracy of one-tenth of the side of a pixel, will be specified in the sequel of the text of the measuring stage.
In order to reduce the processing time for the determination of the position of a star, one can first of all determine its approximate position so that, for the measurement, only a limited zone around the image spot must be considered. This preliminary stage, the search stage, is effected in a portion of the image zone of the detection matrix, denoted search window, whose dimensions depend on the estimated position of the star and the associated uncertainty.
This search window, which constitutes a square or rectangular subassembly of the detection matrix of the order of some hundreds of pixels, the still further limited zone whose position is derived from the search stage and to which the calculating algorithms are applied, is denoted the measuring field. The measuring field which itself forms a generally square subassembly of the search window, is made as small as possible and, typically, can vary from a 4 by 4 pixel pattern to a square pattern which may contain several hundreds of pixels. Calculating the position in the measuring stage can be effected, for example, in the manner described in the French patent application No. 2 625 396, corresponding to U.S. Pat. No. 4,979,221, wherein a memory is accessed which contains the precalculated positions of the centre of the luminous signal as a function of the signal levels found over the 5 pixels nearest to the centre. The precision obtained in accordance with this procedure and the device associated therewith is equal to one quarter of the side of a pixel.